Engines may be configured with exhaust gas recirculation (EGR) systems to divert at least some exhaust gas from an engine exhaust manifold to an engine intake manifold. By providing a desired engine dilution, such systems reduce engine knock, throttling losses, in-cylinder heat losses, as well as NOx emissions. As a result, fuel economy is improved, especially at higher levels of engine boost. Engines have also been configured with a sole cylinder (or cylinder group) that is dedicated for providing external EGR. Therein, all of the exhaust from the dedicated cylinder group is recirculated to the intake manifold. As such, this allows a substantially fixed amount of EGR to be provided to engine cylinders at most operating conditions. By adjusting the fueling of the dedicated EGR cylinder group (e.g., to run rich), the EGR composition can be varied to include species such as hydrogen which improve the EGR tolerance of the engine and result in fuel economy benefits.
While the availability of EGR over a larger operating range provides fuel economy benefits, the fixed EGR rate also reduces the peak torque capability of the engine. In addition, catalyst warm-up may be degraded, particularly after an engine cold-start.
Various approaches may be used to reduce the EGR rate in such dedicated EGR systems during conditions when EGR reduction is required. One example approach shown by Gingrich et al. in US 20120204844 uses a diverter valve for diverting exhaust from the dedicated EGR cylinder to an exhaust location. By redirecting exhaust to a turbine location, peak torque output may be improved. However, the use of diverter valves may be cost prohibitive. In addition, they may have durability issues. Another example approach, shown by Boyer et al. in US20140196703, uses exhaust variable valve timing to direct exhaust from a dedicated EGR cylinder to the intake when EGR is required and direct exhaust away from the intake, towards a turbine, when EGR is not required. In still further examples, the dedicated EGR cylinder may be deactivated by deactivating fuel and spark to the cylinder.
However, the inventors herein have recognized potential issues with the above approaches. As an example, during transitions when exhaust flow from the dedicated EGR cylinder is being directed to the intake or redirected to the exhaust, such as when the dedicated EGR cylinder is being reactivated or deactivated, torque disturbances may be experienced. As such, it may be difficult to reduce the torque disturbances while concurrently maintaining accurate control of other engine operating parameters, such as air-fuel ratio, spark timing, and cam timing. The inventors herein have recognized that transients experienced during deactivation and reactivation of a dedicated EGR cylinder may have a substantially more complex relationship with engine torque output than the deactivation and reactivation of a regular engine cylinder (such as a cylinder that can be deactivated by selective fuel or valve deactivation). This is because in addition to exhaust gas being redirected from the dedicated EGR cylinder to a pre-turbine location, EGR is also being purged from the intake manifold. This results in conflicting torque changes as the exhaust being redirected to the pre-turbine location may increase peak torque while a delay in purging of EGR from the intake manifold results in torque loss from the dedicated EGR cylinder. As an example, even after exhaust from the dedicated EGR cylinder has been redirected away from the intake manifold, and intake airflow has been increased, due to delays in manifold filling, there may be a corresponding delay in purging EGR from the engine intake. As such, until the EGR has sufficiently purged, torque may be lower than desired. At the same time, turbocharger performance may be increased due to the redirection of exhaust to the pre-turbine location. Consequently, throttle adjustments may need to compensate for the balance between torque loss due to EGR and torque gain due to increased flow through the turbine. As another example, when EGR is desired and dedicated EGR operation is re-activated, the same delay in manifold filling may result in lower engine dilution than desired, and a resulting torque excursion. As such, until the EGR has been ramped up to the desired rate, there may be torque unevenness.
In one example, the above issues may be at least partly addressed by a method for an engine comprising: transitioning into and out of dedicated EGR cylinder operation while adjusting each of an intake throttle and an exhaust wastegate in opposing directions. In this way, torque transients incurred while EGR from a dedicated EGR cylinder is ramped-up or down, and while a dedicated EGR cylinder is activated and deactivated, can be decreased.
As an example, an engine system may be configured with a single dedicated EGR cylinder for providing external EGR to all engine cylinders. During conditions where EGR demand is low, such as when transitioning from a lower engine load to a higher engine load, the engine may be transitioned out of dedicated EGR cylinder operation by diverting exhaust from the dedicated EGR cylinder away from the EGR passage and engine intake, and towards the exhaust passage, upstream of an exhaust turbine. By redirecting exhaust away from the EGR passage, engine dilution provided by the dedicated EGR cylinder is reduced. Deactivating the EGR cylinder also results in engine output torque initially decreasing. Then, as the EGR in the intake manifold is used up and replaced with fresh air, the engine output torque increases. To reduce the torque unevenness involved with the transition out of dedicated EGR cylinder operation, during the transition from the lower engine load to the higher engine load, after switching exhaust from the dedicated EGR cylinder towards the exhaust turbine, each of an intake throttle and an exhaust wastegate are modulated to expedite purging of residuals from the intake manifold and refilling of the intake manifold with fresh air. The throttle and wastegate adjustment may be performed when the dedicated EGR cylinder is reactivated during an engine start only after an emission control device reaches a threshold temperature, such as a light-off temperature.
Specifically, during an initial phase of the transition, the throttle is moved from an initial, less open position corresponding to the lower load to a transient, more open position corresponding to a higher load via an overshoot position where the throttle is opened more than required at the final position. In other words, the throttle opening is increased more than required, and then transiently held at the more than required open position before being returned to the final position corresponding to the higher load. At the same time, an exhaust wategate is moved from an initial, more open position corresponding to the lower load to a transient, less open position corresponding to the higher load via an undershoot position where the wastegate is closed more than required at the final position. In other words, the wastegate opening is decreased more than required, and then transiently held at the more than required closed position before being returned to the final position corresponding to the higher load. In some embodiments, spark timing and cam timing may also be concurrently modulated. For example, during the transition out of dedicated EGR cylinder operation, while the intake throttle opening is increased, spark timing may be retarded while intake cam timing may be advanced. Then, when the throttle opening is returned to the final position, spark timing may be advanced back towards MBT while cam timing may be retarded back to a timing corresponding to the higher load.
In this way, external EGR can be varied by diverting exhaust from the dedicated EGR cylinder away from the intake, while reducing torque unevenness during the activating or deactivating using engine actuator adjustments. By subsequently adjusting the intake throttle position, the wastegate position, the spark timing, and the cam timing to “base” positions before the transition is completed, a torque surge anticipated when the EGR is replaced with fresh air can be averted. By reducing torque unevenness during conditions when EGR is ramped in or ramped out from a dedicated EGR cylinder, a smoother transition is enabled and engine performance is improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.